The present invention is directed to the discovery that it is possible to create a first-stage vacuum pump against atmosphere that can isolate atmospheric, light gases, such as helium and hydrogen by preventing the back diffusion (backwards migration) of these gases from the exhaust port of the pump to the inlet port, all as a function of the pump-operation, without the use of additional components such as valves. It is also part of the discovery of the invention that it is possible for the first-stage vacuum pump against atmosphere to provide improved pumping efficiency of light gases, where the percentage of the light gases that enter the pump that is successfully expelled is significantly improved over conventional first- stage vacuum pumps. These discoveries further relate to the specific application of scroll vacuum-pumps, and the configuration-operating parameters that determine the ability of scroll vacuum-pumps to achieve light-gas isolation and efficient light-gas pumping. The present invention relates to the specific application of specifically-configured scroll vacuum-pumps where a minimum background-presence of light gases is required. The present invention is also related to the mechanical configuration and operating parameters that determine the ability of scroll vacuum-pumps to achieve light-gas isolation and efficient light-gas pumping. These mechanical configurations and operating parameters consist of the scroll pumping mechanism tolerances, scroll pumping-mechanism orbiting speed, the vacuum-pressure in the scroll mechanism compression-chambers, the absence of light-gas absorbing materials inside the pump and the number of scroll mechanism compression-chambers between the inlet of the scroll vacuum-pump and its outlet. The optimization of one of these parameters can make an oil-free, scroll-type vacuum-pump outperform conventional, rough vacuum-pumps in respect to light-gas pumping efficiency. Optimization of all of the of the above makes it possible to create a first-stage, scroll vacuum-pump that can provide total, light-gas isolation, and ultra-high performance, light-gas pumping efficiency.
Due to the low atomic mass-weight of light gases, such as helium or hydrogen, it is difficult to efficiently pump these gases with conventional, first-stage vacuum-pumps against atmosphere vacuum, such as oil van or diaphragm rough-vacuum pumps. In addition, both helium and hydrogen are light, fast moving atoms that do not retain the desired directional velocity for efficient pumping, or back-migration isolation, by conventional high-vacuum pumps, such as diffusion, molecular-drag or turbomolecular pumps. This has long created problems for the many vacuum systems that need a low background of light gases, such as high sensitivity helium leak detectors and residual-gas analysis systems, or critical vacuum-processes where light gases are a contaminant. In light-gas measurement systems, back-diffusion of atmospheric light gases through all vacuum pumping stages can create unstable sensor-readings for the quantity of light gas in the vacuum-chamber that is under test. These systems will benefit greatly from the present invention's first-stage roughing pump against atmosphere that can prevent atmospheric, light-gas, back diffusion (backwards migration) from the exhaust port of the pump to the inlet port. These vacuum-systems will also benefit from the present invention, in that it provides high pumping efficiency and high-speed pumping throughput of light gases from the pump-inlet to the pump-exhaust, without the use of light-gas absorbing materials in the pump that will later release the absorbed light gases in bursts that back diffuse to the pump-inlet, which is a fundamental problem associated with prior-art rough-vacuum pumps.
The present invention is directed to the use of a conventional, oil-free scroll-pump as a first-stage roughing pump, in order to isolate the back-flow of gases in a vacuum system. The present invention is directed to the discovery that a scroll-pump has the Capability of completely preventing the back-flow of light gases from the exhaust of the scroll-pump to its inlet, which has especial significance in vacuum systems where it is highly desirable to reduce, or entirely eliminate, such back-flow or back-diffusion.
Vacuums have long been used as an environmental control for experiments and processes. Many times a user wants to remove air, or other gases, from a volume to the lowest level possible, and then fill that volume with a high-purity gas, or gas mixture, for an experiment or manufacturing process. These volumes are called process chambers. Three issues are important: Cleanliness of the containment-vessel, atmospheric leaks into the vessel, and the gases that have not been removed from the process chamber. Hydrogen, which is a highly-reactive gas, is sometimes a major issue in both the manufacturing and the containment of high-purity gases.
Due to the low, atomic mass-weight of light gases, such as hydrogen and helium (atomic mass units 2 and 4, respectively), it is very difficult to evacuate these light gases with conventional vacuum-pumps. The problem is that hydrogen and helium are very light, fast-moving atoms, that do not easily retain the desired directional velocity for effective pumping 23 by conventional, vacuum, diffusion-pumps, turbo-molecular pumps, molecular drag-pumps, etc. These pumps use, as their first stage, a mechanical pump operating in the pressure range of 1.times.10.sup.-2 to 2.times.10.sup.-1 torr, and, in some cases, pressure as great as 30 torr. The first-stage mechanical pump, commonly referred to as foreline or backing pump, is used with a second stage, high-vacuum pump, such as diffusion, turbomolecular, molecular-drag pumps, etc., which are, typically, oil-vane pumps or multi-stage diaphragm pumps. With the latter, the light gases, hydrogen and helium, are absorbed and released by the diaphragm, which is, typically, an elastomeric material, which allows these light, very active atoms to back-flow or backstream, and, thus, to return to the high-vacuum pump, and, then, back-stream through the high-vacuum pump, and, thereby, return to the volume or process chamber that is being evacuated. The higher the concentration of light gases in the high-compression stage of a high-vacuum pump, and in the lines connected to the first-stage foreline-pump and the fore-pump itself, the more the light gases will back-stream through the high-vacuum pump, to return to the vessel that is being evacuated. This back-flow problem is, further, compounded by the fact that diaphragm or membrane pumps have a very low, gas-compression factor.
In the case of an oil-vane pump as the first-stage pump (the type most commonly used), the back-flow of light gases is, further, compounded by the fact that oil is used in the oil-case of the vane-pump. An oil-vane pump uses a stator, which is a stationary volume, in which gases are compressed by a rotor, which is internal to, and revolves in, the stator of the pump. The rotor has slots that are machined through its centerline, in which springs create opposing forces to the vanes and make the vanes contact the walls of the stator. The inlet of the vane-pump allows a volume of gas in the stator to equalize in pressure with the volume being evacuated. As the rotor and vanes rotate, the inlet is isolated, and the trapped volume is compressed and forced through an exhaust valve, thus creating a reduction in gas molecules, and pressure, in the volume being evacuated. The rotor, stator and exhaust valve are all submerged in oil, and mounted in an oil-case. The function of the oil is to lubricate and seal the internal surfaces that are making contact within the stator, namely the rotor, vanes, and exhaust-valve. The oil, thus, lubricates and helps to conduct the heat away from the pumping mechanism, and, also, seals the rotor to the stator, and seals the sliding vanes to both the stator and the end-plates, as well as to the rotor, giving a better seal, and, thus, better compression. The oil also covers the exhaust valve, and aids in the sealing of the exhaust valve. The problem with these oil-vane pumps, however, is that compressed gases form bubbles in the oil, or fluid, and are entrained in the oil, and are, thus, re-injected back into the pumping chamber for lubrication and sealing. These bubbles burst in the pump chamber, thus allowing the light, previously-pumped gases to be reintroduced into the oil- vane pump, to, thus, backstream into the high-compression area of the high-vacuum pump, which causes a higher concentration of the gases in the high-vacuum pump from which these gases may have come in the first place. The greater back-streaming through the high-vacuum pump may allow even higher numbers of these light atoms to return to the volume that is being evacuated, as these light gases will enter the exhaust of the vane- pump from the ambient.
The motion of all atoms and molecules is based on the statistical thermodynamics, and, specifically, motion is determined by the mean-free path of an atom or molecule. The mean- free path is the distance that an atom or molecule can travel before colliding with another atom or molecule, or with the walls that contain them. These collisions impede its travel in back-streaming, or in its pathway to being exhausted to atmosphere by the pump. In viscous flow, there is a pressure differential, with the more negative, or vacuum, pressure being at the pump inlet, whereby, a large number of gas molecules and atoms are entrained, or constrained by the walls and other atoms or molecules. Turbulent flow shortens the mean-free path of an atom or molecule traveling away from the pump that is trying to capture and exhaust them to atmosphere, thus improving the vacuum-pressure. There is a transition phase of gas flow as the pressure differential, or delta P, becomes less and less at the vacuum pump, as gas molecules and atoms are removed. As the gases continue to be removed, the pressure differential virtually becomes zero, and the mean-free path becomes longer and longer, as gas continues to be expelled by the pump. This final phase of gas flow is known as molecular flow, with no pressure differential, and a longer mean-free path before the gas molecules run into each other. Gas-flow becomes random collisions, where the likelihood that a molecule will move toward the pump, be captured, and be exhausted to ambient, with the concomitant lowering of vacuum pressure, is considerably reduced. It is at this phase where light, active gases, such as hydrogen and helium, become a significant problem.
One area where back-flow has been a considerable problem has been in helium-leak detection systems. In these systems, the more helium that can removed from the analyzer or mass-spectrometer cell, the greater sensitivity and the lower the helium-background, which means more net sensitivity. Background is defined as ionized atoms and molecules that strike the collector that are not helium. Sensitivity equals the percentage of only mass 4 ions that strike the collector. Thus, true helium-sensitivity equals sensitivity minus the background signal.
Another area where back-flow is a problem is in a residual gas analyzer, which is a device used in vacuum technology to ascertain the gas species and their concentrations in a vacuum chamber, by measuring their atomic mass unit (AMU). The signal from the sensor indicates the partial-vacuum pressure of a particular AMU or specific gas specie. Although a residual gas analyzer can read AMU's from 1-400, the more commonly used are 1-100 AMU. The main interest to a vacuum technologist are 1-50 AMU. Hydrogen is mass 2 or 2 AMU, helium 4, water vapor comprises 16, 17 and 18, nitrogen 28, oxygen 32, etc.
The residual gases in a vacuum-chamber that are difficult to remove are hydrogen, helium and water vapor. While water vapor is easy to pump or capture, the binding energy to a surface in vacuum is very great. Water molecules cannot be pumped, or captured, until it leaves the surface and gets to the trap or pump. Thus, water vapor is a major problem in attaining a low vacuum-pressure in a reasonable time-frame. Light, active gases, although they arrive at the pump in a much shorter time-frame, are very difficult to compress and eject out to atmosphere. As stated previously, they back-stream through commonly-used pumps, and return to the chamber, whence they have to be pumped away over and over again, until they are finally expelled by the mechanical pump to atmosphere. To add to the problem, high-vacuum components and chambers are typically stainless steel, which continually produce hydrogen. Also, most materials are permeable to hydrogen and helium. Sealing materials are typically elastomers, as is the diaphragm of a diaphragm-pump. These materials have high permeation-rates for light gases into vacuum, giving an even greater light-gas load that must be pumped.